1. Field of the Invention
The present invention relates generally to optical communications, and more specifically to devices and methods providing dispersion compensation and Raman amplification of an optical signal.
2. Technical Background
As the bit rates of optical communications systems increase, the deleterious effects of dispersion in the optical fibers used in long-distance transmission become increasingly important. Dispersion causes optical pulses to spread out in time; the lower wavelength components of the pulse travel along the fiber at a different rate than do the higher wavelength components of the pulse. Typically, long-distance transmission fibers (e.g. LEAF®, available from Coming Incorporated of Corning, N.Y.) have a small but non-negligible positive dispersion, causing the higher wavelength components to arrive at a network node before the lower wavelength components. This temporal spreading can cause loss of signal fidelity and an increase in bit error rate.
Conventional methods of dispersion compensation use dispersion compensating fiber to reverse the effects of dispersion in the transmission fiber. Dispersion compensating fiber typically has a large negative dispersion to counteract the positive dispersion of the transmission fiber. In one type of conventional dispersion compensating device, a dispersion compensating fiber is packaged on a spool in a module. The length and dispersion properties of the dispersion compensating fiber are chosen to balance the dispersion of the span of transmission fiber to which it is coupled. A positively chirped optical signal from the transmission fiber is propagated through the dispersion compensating fiber, and the negative dispersion of the dispersion compensating fiber removes the positive chirp from the optical signal.
Stimulated Raman scattering is a well-known nonlinear phenomenon that can allow many conventional optical fibers to provide broadband amplification. A weak signal in the 1550–1620 nm wavelength range can be amplified by propagation in an optical fiber with strong pump radiation (typically in the 1430–1480 nm wavelength range). The higher energy pump radiation scatters off atoms in the optical fiber core, loses some energy to those atoms, and propagates down the fiber with the same wavelength as the signal. The amount of amplification provided by stimulated Raman scattering in an optical fiber is proportional to the Raman scattering coefficient of the material of the optical fiber core, and is inversely proportional to the product of the fiber's effective area and its absorption loss. One conventional method to achieve signal amplification in a dispersion compensating device is to directly pump the dispersion compensating fiber. Typical dispersion compensating fibers are effective Raman gain media due to their nonlinear nature and small effective area.
Raman pumping of the dispersion compensation fiber is not always sufficient to provide a desired amount of gain. For example, dispersion compensating devices designed to compensate for relatively short spans of transmission fiber have relatively short lengths of dispersion compensating fiber. The short length of the dispersion compensating fiber in such devices limits the amount of Raman gain that can be achieved.